The present invention is directed to a novel N.sup.4 -protected, 2'-O-methyl cytidine monomer and methods for its preparation.
Oligonucleotides which include 2'-O-alkyl nucleosidyl units have been proposed as useful antisense probes for studying RNA processing. (Iribarren, A.M. et al., Proc. Natl. Acad. Sci. (USA) 87:7747-7751 (1990).
The 2'-O-methyl oligonucleotides have also been reported to inhibit certain mRNA processing events in vitro. See, e.g., Cotten, M, et al., Nucl. Acids. Res. 19(10):2629-2635 (1991).
Synthesis of 2'-O-methyl oligonucleotides has been reported using certain protected nucleotide monomers.
Since certain of the nucleotide monomers have reactive groups, such as the N.sup.4 amino in cytidine and the N.sup.6 amino in adenosine, these groups must be protected during synthesis of the Oligomer using protecting groups which are removable under non-adverse conditions after synthesis of the Oligomer is complete. Cytidine monomers having N.sup.4 -benzoyl protecting groups are conventionally used in oligonucleotide synthesis. See, e.g., Inoue, H., et al., Nucl. Acids Res. 15(15):6131-6147 (1987).
N.sup.4 -benzoyl-protected 2'-O-methyl cytidine has been found to undergo unwanted side reactions, particularly a transamination reaction, when treated with certain deprotecting reagents, in particular ethylenediamine which is used as a deprotecting reagent in the synthesis of methylphosphonate Oligomers. This transamination reaction has been found to result in decreased yields of methylphosphonate Oligomers.